OSAT Biological Sciences (110) Practice Exam 2025 - Free Biology Practice Questions and Study Guide

Guard cells are specialized cells that surround and control the opening and closing of stomata, which are the small openings on the surface of leaves. The primary function of guard cells is to regulate gas exchange and water loss in plants. When guard cells swell with water (becoming turgid), they curve and open the stomatal pore, allowing carbon dioxide to enter the leaf for photosynthesis and facilitating the release of oxygen. Conversely, when the guard cells lose water and become flaccid, they close the stomata to prevent excessive water loss, especially under drought conditions.

This regulation is critical for maintaining homeostasis within the plant and ensuring efficient photosynthesis. The other cell types mentioned do not play a role in the regulation of stomatal openings: parenchyma cells are involved in storage and photosynthesis, phloem cells are responsible for transporting organic nutrients, and xylem cells transport water and minerals. Thus, guard cells are uniquely suited to perform the essential function of controlling stomatal dynamics.